avoid warning in noCancel with non-raising future (status-im#540)

chronos/internal/asyncfutures.nim

@@ -1031,19 +1031,24 @@ proc noCancel*[F: SomeFuture](future: F): auto = # async: (raw: true, raises: as
   let retFuture = newFuture[F.T]("chronos.noCancel(T)",
                                 {FutureFlag.OwnCancelSchedule})
   template completeFuture() =
+    const canFail = when declared(InternalRaisesFutureRaises):
+      InternalRaisesFutureRaises isnot void
+    else:
+      true
+
     if future.completed():
       when F.T is void:
         retFuture.complete()
       else:
         retFuture.complete(future.value)
-    elif future.failed():
-      when F is Future:
-        retFuture.fail(future.error, warn = false)
-      when declared(InternalRaisesFutureRaises):
-        when InternalRaisesFutureRaises isnot void:
-          retFuture.fail(future.error, warn = false)
     else:
-      raiseAssert("Unexpected future state [" & $future.state & "]")
+      when canFail: # Avoid calling `failed` on non-failing raises futures
+        if future.failed():
+          retFuture.fail(future.error, warn = false)
+        else:
+          raiseAssert("Unexpected future state [" & $future.state & "]")
+      else:
+        raiseAssert("Unexpected future state [" & $future.state & "]")
 
   proc continuation(udata: pointer) {.gcsafe.} =
     completeFuture()

tests/testmacro.nim

@@ -519,7 +519,7 @@ suite "Exceptions tracking":
 
     noraises()
 
-  test "Nocancel errors":
+  test "Nocancel errors with raises":
     proc testit {.async: (raises: [ValueError, CancelledError]).} =
       await sleepAsync(5.milliseconds)
       raise (ref ValueError)()
@@ -535,6 +535,36 @@ suite "Exceptions tracking":
 
     noraises()
 
+  test "Nocancel with no errors":
+    proc testit {.async: (raises: [CancelledError]).} =
+      await sleepAsync(5.milliseconds)
+
+    proc test {.async: (raises: []).} =
+      await noCancel testit()
+
+    proc noraises() {.raises: [].} =
+      let f = test()
+      waitFor(f.cancelAndWait())
+      waitFor(f)
+
+    noraises()
+
+  test "Nocancel errors without raises":
+    proc testit {.async.} =
+      await sleepAsync(5.milliseconds)
+      raise (ref ValueError)()
+
+    proc test {.async.} =
+      await noCancel testit()
+
+    proc noraises() =
+      expect(ValueError):
+        let f = test()
+        waitFor(f.cancelAndWait())
+        waitFor(f)
+
+    noraises()
+
   test "Defect on wrong exception type at runtime":
     {.push warning[User]: off}
     let f = InternalRaisesFuture[void, (ValueError,)]()